Electrodeposition of zirconium diboride

ABSTRACT

Coherent coatings of zirconium diboride are deposited electrolytically from a melt containing at least one fluoride of potassium, rubidium or cesium, at least one fluoride of an element higher in the electromotive series than zirconium and boron, at least one fluoride of zirconium, and boron trioxide. The process employs an anode of zirconium or zirconium diboride, and an electrically conductive base material as a cathode. The temperature of the electrolytic melt is maintained in excess of 750*C., and the oxygen to boron molar ratio in the melt is maintained below 1.75.

United States Patent 1 Mellors et a1.

[ 1 Jan.30, 1973 [54] ELECTRODEPOSITION OF ZIRCONIUM DIBORIDE [751 Inventors: Geoffrey W. Mellors, Strongsvill; Seymour Senderoff, Fairview Park, both of Ohio [73] Assignee: Union Carbide Corp., New York,

[22] Filed: June 8, 1970 [21] App1.No.: 44,651

[52] U.S. Cl. ...204/3, 204/39, 204/61 [51] Int. Cl ..C23b 7/00, B011; H00 [58] Field of Search ..204/39, 3, 61

[56] References Cited UNITED STATES PATENTS 3,600,284 8/1971 Martinez ..204/39 3,479,158 11/1969 Cook ..204/39 Primary Examiner-.lohn H. Mack Assistant Examiner-R. L. Andrews AttorneyPaul A. Rose, John F. Hohniann, Charles J. Metz, Gerald R. OBrien and William R. Moran [57] ABSTRACT Coherent coatings of zirconium diboride are deposited electrolytically from a melt containing at least one fluoride of potassium, rubidium 0r cesium, at least one fluoride of an element higher in the electromotive series than zirconium and boron, at least one fluoride of zirconium, and boron trioxide. The process employs an anode of zirconium or zirconium diboride, and an electrically conductive base material as a cathode. The temperature of the electrolytic melt is maintained in excess of 750C, and the oxygen to boron molar ratio in the melt is maintained below 1.75.

15 Claims, N0 Drawings ELECTRODEPOSITION OF ZIRCONIUM DIBORIDE The invention relates to the electrodeposition of zirconium diboride (ZrB as coherent coatings or electroformed objects.

Zirconium diboride has been successfully electrodeposited in coherent coatings only in the process utilizing an all fluoride electrolytic melt that has been described by the inventors herein (U.S. Pat. No. 3,444,058, patented May 13, 1969, and .l. Electrochem. Soc., 1 13, 60, 1966). In that process, zirconium diboride was electrodeposited from a system utilizing an all fluoride melt, for instance, a melt containing lithium fluoride, potassium fluoride, potassium fluozirconate, and potassium fluoborate. Zirconium metal was used as the anode in this system. The process was successful in producing a coherent coating of zirconium diboride on a substrate. However, there were certain disadvantages. At the anode, four faradays of electricity dissolve one mole of zirconium, while ten faradays are required to deposit one mole of zirconium diboride at the cathode. There is therefore a net increase of zirconium in the bath, with the result that the composition of the bath gradually changes. In addition, the use of KBF, as the source of boron in the system leads to evaporation of BF from the system, with consequent reduction in the concentration of available boron.

In order to overcome the first enumerated disadvantage (that is, the gradual increase of zirconium in the melt), attempts were made to use a zirconium diboride anode in the process. While the deposits thus produced were zirconium diboride, they were of poor appearance, and were rough and darkened over large portions of their area. It was then found that zirconium diboride could be electrodeposited from a melt comprising the fluorides of lithium, sodium, potassium, and zirconium, utilizing a zirconium anode and bubbling boron trifluoride through the melt with an argon carrier. However, agitation caused by the passage of the gases through the melt was sufficient to produce rougher deposits than are desirable. While baffling the system in order to reduce the turbulence could perhaps eliminate the roughness of the deposits, this path was not pursued further by the inventors.

It was attempted to deposit zirconium diboride from a lower melting system consisting of 32.4 weight per cent potassium fluoride and 67.6 weight per cent zirconium tetrafluoride, the melting point of which is only 390C. With the addition of potassium fluoborate to this low melting system, the attempt to electrodeposit zirconium diboride at a temperature of 400 to 425C. (at which temperature, volatility of BB, would be less of a problem) resulted in the production of a mixture of zirconium and zirconium diboride powders rather than the desired coherent deposit of zirconium diboride.

'In the process described in U.S. Pat. No. 3,444,058 and the J. Electrochem. Soc. article cited above, the inventors had found that the presence of oxygenated materials in an electrodeposition melt was deleterious and prevented the obtaining of a coherent coating. In accordance with the present invention, however, it has been discovered that a coherent deposit of zirconium diboride can be successfully electrodeposited from a melt containing boron trioxide (B wherein the con tent of the boron trioxide (and hence oxygen) of the melt is controlled between certain critical limits.

It is a main object of the present invention to provide a process for electrodepositing coherent deposits of zir conium diboride.

It is another object of this invention to provide a method for electrodepositing coherent coatings of zirconium diboride wherein the method has substantially eliminated certain of the problems of the prior art processes.

Further objects of the invention will be apparent from a reading of the detailed specification which follows.

The invention provides a process for producing coherent deposits of zirconium diboride. The process is an electrolytic process which utilizes an electrolytic melt containing a base melt, at least one fluoride of zirconium, and boron trioxide. The process utilizes as the anode either zirconium or zirconium diboride, and as the cathode, an electrically conductive base material. The process is carried out while maintaining the electrolytic melt at a temperature in excess of 750C., and while maintaining the oxygen to boron molar ration in the electrolytic melt within certain carefully defined limits, which are discussed more fully below.

The base melt in the electrolytic melt is at least one fluoride of potassium, rubidium, or cesium, and at least one fluoride of other elements higher in the electromotive series than zirconium and boron. A preferred base melt is composed of the fluorides of lithium, sodium, and potassium, and more preferably, it is composed of the eutectic mixture of these materials. Said eutectic mixture contains 29.25 weight per cent of lithium fluoride, l 1.70 weight per cent of sodium fluoride, and 59.05 weight per cent of potassium fluoride, and it has a melting point of 454C. This eutectic mixture is known in the art as Flinak.

The electrolytic melt also contains at least one fluoride of zirconium. Preferably, zirconium tetrafluoride (ZrF is employed. Other zirconium fluorides can be employed if desired, such as potassium fluozirconate, or the like. The fluoride of zirconium is present in the electrolytic melt in an amount sufficient to provide from about 5 to about 30 weight per cent of the simple fluoride of zirconium (i.e., zirconium tetrafluoride) in the electrolytic melt. It is also required that the zirconium in the electrolytic melt be maintained in the tetravalent state during production of the deposit of zirconium diboride. Preferably, the electrolytic melt contains from about 6 to about 10 weight per cent of zirconium tetrafluoride.

The electrolytic melt also contains boron trioxide in an amount from about 3.2 to about 32 weight per cent, based upon the weight of the electrolytic melt. Preferably, the said melt contains at least about 5.7 weight per cent of boron trioxide, and more preferably from about 10 to about 12 weight per cent of boron trioxide.

The oxygen to boron molar ratio in the melt should be maintained below about 1.75. As a practical matter, when the anode employed in the electrolytic process of the invention is composed of zirconium diboride, the electrolytic melt will automatically maintain its oxygen to boron molar ratio within the desired range. However, when an anode composed of zirconium is employed, the oxygen to boron ratio is maintained below the said 1.75 limit by periodic additions of boron trioxide. (These additions are needed because the boron content of the melt is gradually depleted when a zirconium rather than zirconium diboride anode is used.) For example, it has been found that approximately 3.5 grams of B should be added for each ampere-hour of electrolysis completed since either the initial charge of B 0 in the melt or subsequent make up additions of B 0 Additions are normally required after about 20 ampere-hours of electrolysis for each kilogram of melt. For instance, given a l kilogram bath and an electrolysis at the rate of 5 amperes per hour, an addition of 70 grams of B 0 is made after 4 hours of operation. With a 2 kilogram bath at the same rate of electrolysis, 140 grams of B 0 are added after 8 hours of operation.

The temperature of the electrolytic melt should be maintained in excess of 750C. during the deposit of the desired zirconium diboride. For instance, as the melt temperature is increased to about 800C., a bright coherent deposit of zirconium diboride is obtained. Theoretically, there is no maximum temperature above which the process of the invention cannot be carried out, given the limitations of melting point, volatility, etc., of the equipment and/or the materials in the melt. However, as a practical matter, corrosion of the equipment begins to become a problem at temperatures much above 900C. Therefore, it is rare that the process of the invention will be carried out at temperatures in excess of 950C. Preferably, the process of the invention is carried out at temperature within the range of from about 800 to about 900C., and more preferably, from about 800 to about 820C.

The electrodeposition step is preferably carried out in an inert, non-oxidizing atmosphere such as argon, neon, helium, or the like. If an inert gas is employed, it may be at the pressure above or below atmospheric pressure, as long as it is substantially inert with respect to the melt and the zirconium diboride to be deposited. The container for the melt may be made of any material which has no deleterious effect on the melt or the deposited zirconium diboride and is not attacked by the melt during operation. Such materials include carbon (graphite), nickel, copper, stainless steel, monel, Inconel 600, and the like.

The electrolytic process is normally carried out such that the current density at the cathode is maintained within the range of from about 5 to about 100 milliamperes per square centimeter during production of the deposit, and preferably, the cathode current density is maintained below about 50 milliamperes per square centimeter. As a general rule, it is desirable to maintain the anode current density within a range of from about 1 to about 10 milliamperes per square centimeter, and preferably, less than about 5 milliamperes per square centimeter. The factors to be considered in maintaining the electrolytic process within the above mentioned limits are well known to those of ordinary skill in the art.

As has been mentioned above, the anode can be either zirconium diboride or zirconium. Preferably, the anode is zirconium diboride. Of course, the anode should be as pure as possible in order to avoid introduction of impurities into the melt.

The anode can be solid ZrB or zirconium or it can be zirconium or ZrB, coated on an inert material such as graphite.

Any conducting substrate which does not melt at operating temperature and is not attacked by the melt can be used as the cathode. Specific illustrative cathode materials include copper, stainless steel, graphite, tantalum, and the like.

The process of this invention may be used to electroplate or electroclad any of the above indicated cathode substrate materials with a deposit of zirconium diboride. The provision of a coating of zirconium diboride enhances the basic utility of the substrate material to the extent that the extremely high corrosion resistance at high temperatures of the zirconium diboride is added to the basic properties of the substrate.

An electroformed object of pure ZrB can be made by removing the substrate after cladding. The removal of the substrate can be accomplished by mechanical means such as drilling or grinding, by chemically dissolving the substrate, by melting the substrate, or the like.

The following non-limiting examples illustrate the process of the invention:

EXAMPLE l An electrolytic melt was prepared containing weight per cent Flinak (the eutectic mixture of LiF, NaF and KP), 10 weight per cent K ZrF and 10 weight per cent B 0 The total weight of the melt was 950 grams. The melt was contained in a nickel crucible. A compacted slug (1 inch diameter by 4 inches long) of ZrB- (purchased from the Carborundum Company) was employed as the anode. During most of the electrolysis, the cathode was copper, although two different turbine blade alloys and stainless steel were employed as the cathode at intervals during the run. (This demonstrates that the composition of the cathode is not narrowly critical). The melt was electrolyzed at a rate such that the cathode current density was maintained at 5mA/cm During most of the run, the current was mA, although with larger cathodes, a current as high as 500 mA was employed. The anode current density varied from about 0.5 to about 2.5 mA/cm The electrolysis was continued until 90.35 amp-hrs of electricity had been passed through the cell. This corresponds to the dissolution and deposition of 38.4 g ZrB (l amp-hr deposits 0.425 g Zrl3 During the electrolysis, the melt was maintained at about 800-81 0C. The initial melt contained 32.18 g Zr and 22.0 g B, so it may be seen that while almost complete turnover of the zirconium has been achieved (38.4 g ZrB contains 31.03 g Zr and 7.37 g B) some time remains before the same can be said for the boron. The reason for the relatively slow rate of turnover is that it is preferred that the anode current density be of the order of 5 mA/cm to obtain satisfactory dissolution. Given the present anode together with the geometry of the small laboratory cells, it has been necessary to operate at a slower rate than would be possible with larger equipment. In the latter case one would arrange for a large anode area and obtain consequent low anode current density while a reasonable plating rate would be obtained with a cathode current density of about 50 mA/cm (i.e., anode: cathode area ratio of 10:1).

Chemical analysis of a typical ZrB coating deposited on the cathodes showed 79.85 percent Zr and 19.80 percent boron compared with the theoretical values of 80.82 percent and 19.18 percent, respectively. X-rays of many samples showed excellent matches for both d spacings and intensities with the standard ASTM card for ZrB There were no extraneous lines in the patterns. In all cases, coherent deposits of ZrB were obtained on the cathode substrates.

EXAMPLE 2 A melt of 80 weight per cent Flinak, 8 weight per cent ZrF and 12 weight per cent B 0 contained in a graphite crucible is electrolyzed. The total weight of the melt initially is about 1300 grams, the anode is zirconium, and the cathode is nickel. The melt temperature is maintained at about 800C.-810C. during the electrolysis. A current density of about mA/cm is maintained at both electrodes throughout the run.

A coherent deposit of ZrB is obtained on the cathode until about 28 ampere-hours of electricity have been passed through the system. Then, compacted powder deposits of ZrB are produced instead of a coherent coating. 100 Grams of B 0 is then added to the melt, and a coherent deposit of ZrB is again made on the cathode. Four separate successive additions of 100 grams of B 0 are made in order to revive the system after about 28 ampere-hours of electricity have passed through the system since either the initial charge or the last addition of B 0 What is claimed is;

1. Process for producing coherent deposits of zirconium diboride which comprises electrolyzing, in an inert atmosphere, an electrolytic melt which consists essentially of:

i. a base melt of:

a. from about 10 to about 90 weight per cent of at least one fluoride of potassium, rubidium, or cesium; and

b. the balance of the base melt being at least one fluoride of other elements higher in the electromotive series than zirconium and boron;

ii. at least one fluoride of zirconium in an amount sufficient to provide from about 5 to about 30 weight per cent of the simple fluoride of zirconium, the percentage being based upon the weight of the electrolytic melt, wherein the zirconium is maintained solely in the tetravalent state during production of said deposit; and

iii. boron trioxide in an amount of from about 3.2 weight per cent to about l2 weight per cent, based upon weight of the electrolytic melt; wherein the anode employed in said process is zirconium or zirconium diboride, wherein the cathode is an electrically conductive base material, and wherein zirconium diboride is formed on said base material as a coherent deposit; provided that the temperature of said electrolytic melt is maintained in excess of 750C. and the oxygen to boron molar ratio in said electrolytic melt is maintained below 1.75, during production of said deposit.

2. Process of claim 1 wherein the proportion of boron trioxide in the electrolytic melt is from about 5.7 weight per cent to about 12 weight per cent.

3. Process of claim 1 wherein the base melt consists essentially of the fluorides of lithium, sodium, and

potassium.

4. Process of claim 1 wherein the base melt consists essentially of the eutectic mixture of the fluorides of lithium, sodium, and potassium.

5. Process of claim 1 wherein the electrolytic melt consists essentially of sodium fluoride, potassium fluoride, lithium fluoride, zirconium tetrafluoride, and boron trioxide.

6. Process of claim 5 wherein the electrolytic melt consists essentially of the eutectic mixture of the fluorides of lithium, sodium, and potassium, about 6 to 10 weight per cent of zirconium tetrafluoride, and about 10 to 12 weight per cent of boron trioxide, the balance being said eutectic mixture, and the percentages being based upon total weight of the electrolytic melt.

7. The process of claim 5 wherein the anode is zirconium and wherein the oxygen to boron ratio is maintained below 1.75 by periodic additions of boron trioxide.

8. Process of claim 1 wherein the melt is maintained at a temperature in the range of from about 800C. to about 900C. during production of said deposit.

9. Process of claim 1 wherein the cathode current density is maintained within the range of from about 5 to about 100 milliamperes per square centimeter during production of said deposit.

10. Process of claim 9 wherein the cathode current density is maintained below 50 milliamperes per square centimeter during production of said deposit.

11. Process of claim 9 wherein the anode current density is not more than about 5 milliamperes per square centimeter.

12. Process of claim 1 wherein said process comprises electrolyzing an electrolytic melt consisting essentially of:

i the fluorides of potassium, sodium, and lithium;

ii zirconium tetrafluoride or potassium fluozirconate;

and iii boron trioxide; wherein the anode is zirconium diboride and the cathode is an electrically conductive base material, to form on said base material a coherent zirconium diboride deposit, the electrolyzing being carried out such that the anode current density does not exceed about 5 milliamperes per square centimeter; provided that during the production of said deposit the temperature of the electrolytic melt is maintained at a temperature in the range of from about 800C. to about 900C, and the oxygen to boron molar ratio in said electrolytic melt is maintained below about 1.75.

13. The process of claim 1 wherein the cathode is copper, stainless steel, graphite, or tantalum.

14. The process of claim 1 wherein the said coherent deposit of zirconium diboride is removed from said base material, thereby producing a substantially pure zirconium diboride object.

15. Process of claim 1 wherein the electrolytic melt consists essentially of sodium fluoride, lithium fluoride, potassium fluoride, boron trioxide, and potassium fluozirconate. 

1. Process for producing coherent deposits of zirconium diboride which comprises electrolyzing, in an inert atmosphere, an electrolytic melt which consists essentially of: i. a base melt of: a. from about 10 to about 90 weight per cent of at least one fluoride of potassium, rubidium, or cesium; and b. the balance of the base melt being at least one fluoride of other elements higher in the electromotive series than zirconium and boron; ii. at least one fluoride of zirconium in an amount sufficient to provide from about 5 to about 30 weight per cent of the simple fluoride of zirconium, the percentage being based upon the weight of the electrolytic melt, wherein the zirconium is maintained solely in the tetravalent state during production of said deposit; and iii. boron trioxide in an amount of from about 3.2 weight per cent to about 12 weight per cent, based upon weight of the electrolytic melt; wherein the anode employed in said process is zirconium or zirconium diboride, wherein the cathode is an electrically conductive base material, and wherein zirconium diboride is formed on said base material as a coherent deposit; provided that the temperature of said electrolytic melt is maintained in excess of 750*C. and the oxygen to boron molar ratio in said electrolytic melt is maintained below 1.75, during production of said deposit.
 2. Process of claim 1 wherein the proportion of boron trioxide in the electrolytic melt is from about 5.7 weight per cent to about 12 weight per cent.
 3. Process of claim 1 wherein the base melt consists essentially of the fluorides of lithium, sodium, and potassium.
 4. Process of claim 1 wherein the base melt consists essentially of the eutectic mixture of the fluorides of lithium, sodium, and potassium.
 5. Process of claim 1 wherein the electrolytic melt consists essentially of sodium fluoride, potassium fluoride, lithium fluoride, zirconium tetrafluoride, and boron trioxide.
 6. Process of claim 5 wherein the electrolytic melt consists essentially of the eutectic mixture of the fluorides of lithium, sodium, and potassium, about 6 to 10 weight per cent of zirconium tetrafluoride, and about 10 to 12 weight per cent of boron trioxide, the balance being said eutectic mixture, and the percentages being based upon total weight of the electrolytic melt.
 7. The process of claim 5 wherein the anode is zirconium and wherein the oxygen to boron ratio is maintained below 1.75 by periodic additions of boron trioxide.
 8. Process of claim 1 wherein the melt is maintained at a temperature in the range of from about 800*C. to about 900*C. during production of said deposit.
 9. Process of claim 1 wherein the cathode current density is maintained within the range of from about 5 to about 100 milliamperes per square centimeter during production of said deposit.
 10. Process of claim 9 wherein the cathode current density is maintained below 50 milliamperes peR square centimeter during production of said deposit.
 11. Process of claim 9 wherein the anode current density is not more than about 5 milliamperes per square centimeter.
 12. Process of claim 1 wherein said process comprises electrolyzing an electrolytic melt consisting essentially of: i the fluorides of potassium, sodium, and lithium; ii zirconium tetrafluoride or potassium fluozirconate; and iii boron trioxide; wherein the anode is zirconium diboride and the cathode is an electrically conductive base material, to form on said base material a coherent zirconium diboride deposit, the electrolyzing being carried out such that the anode current density does not exceed about 5 milliamperes per square centimeter; provided that during the production of said deposit the temperature of the electrolytic melt is maintained at a temperature in the range of from about 800*C. to about 900*C., and the oxygen to boron molar ratio in said electrolytic melt is maintained below about 1.75.
 13. The process of claim 1 wherein the cathode is copper, stainless steel, graphite, or tantalum.
 14. The process of claim 1 wherein the said coherent deposit of zirconium diboride is removed from said base material, thereby producing a substantially pure zirconium diboride object. 